The continuous life-long production of all mature blood cells in the circulation and hematopoietic tissues is contingent on hematopoietic stem cells (HSC). These rare cells reside in the bone marrow of the adult and are the clinically important cells in transplantation therapies for blood-related diseases and leukemias. The manipulation of limited numbers of adult HSCs has been difficult, since ex vivo expansion of these cells has not yet been achieved. Despite the identification and use of many hematopoietic growth factors, HSCs can at best be maintained, not expanded. We hypothesized that at some early stage in ontogeny HSCs are generated and expanded in a unique embryonic microenvironment. An understanding of the developmental processes leading to HSC emergence in the embryo should provide novel insights and improved methods for the ex vivo expansion of HSCs for clinical use. To this end, our long-term research objective has been to determine the cellular and molecular mechanisms by which HSCs are generated/expanded within the mammalian embryo. We have shown that the mouse aorta-gonad-mesonephros (AGM) region autonomously generates the first fully potent HSCs in close association with the aortic endothelium. We have demonstrated that Runxl and GATA-2 transcription factors are required for the generation and expansion of these HSCs. However, more factors have yet to be identified. In various preliminary studies of the AGM microenvironment, IL-3, BMP-4 and hedgehog factors have been implicated as effectors of AGM HSC emergence, but these factors must be further characterized. Thus, we will examine each factor for its role in the induction and expansion of HSCs in the midgestation mouse aorta. Using classical embryologic methods, we will deliver factors to whole cultured mouse embryos by bead/cell pellet implantation and transgenesis, to determine their effect on AGM HSCs. In similar factor manipulation experiments, we aim to establish the lineage relationships of precursors to AGM HSCs by dye marking of aortic endothelium. We will also investigate the long-term fate of the HSCs generated in the mouse AGM by Cre-lox recombination technology. If indeed embryonic aorta-derived HSCs migrate and colonize the adult bone marrow, our insights into their generation and expansion should lead to novel methods for clinical HSC expansion and thus, improve transplantation therapies.